Create a volumetric cone shape in eevee?
Thanks to  Inigo Quilez, here's a signed-distance function from the surface of a cone of a given pitch angle. 

float sdCone( vec3 p, vec2 c )
{
  // c is the sin/cos of the angle
  float q = length(p.xy);
  return dot(c,vec2(q,p.z));
}


I would have done it another way, but this is much simpler... it boils down to the distance from a point to a line.(The vector projection proof)



Here, the cone is in the space of an Empty, so you can move it around. You set the angle at the 'Angle' input in radians, and can adjust the scale of the cone with the 'Threshold' setting, and the density inside it by multiplying the result.



I tested it on a stack of planes.. it is filling the volume, with the 'Less Than' (the threshold).. if you use a 'Compare' instead, you can capture just a shell.